This invention relates to a drum brake device that functions as a leading-trailing (LT) type when the service brake is applied, and as a duo-servo (DS) type when the parking brake is applied. More specifically, it relates to a drum brake device in which the configuration facilitates the assembly of the device.
This type of drum brake device has been disclosed, for example, in U.S. Pat. No. 5,275,260, and is explained with reference to FIG. 20 and FIG. 21. A hydraulic cylinder b and an anchor block c are provided on the upper and lower segments of the back plate a respectively. The upper and lower ends of a pair of left and right brake shoes, d, e, engage the hydraulic cylinder b and the anchor block c, respectively. A first shoe return spring f and a second shoe return spring g are stretched between the lower and upper ends f the brake shoes d, e respectively. The parking brake lever h is pivoted on the lower segment of the left brake shoe d. A pivot lever i is pivoted such that its central segment can swing on the central segment of the right brake shoe e. A rod j is mounted horizontally between the brake shoes d, e in the vicinity of the anchor block c. The left end of the rod j engages the brake shoe d and the parking brake lever h. The right end of the rod j engages the brake shoe e and the lower segment of the pivot lever i, respectively.
A screw-type shoe clearance adjustment device k is mounted horizontally between the brake shoes d, e in the vicinity of the hydraulic cylinder b. The left end of the shoe clearance adjustment device k engages brake shoe d, and the right end engages brake shoe e and the upper segment of the pivot lever i respectively. An adjustment lever l is pivotable on the web of the right brake shoe e. One arm of the adjustment lever l engages the right end of the shoe clearance adjustment device k, and another arm engages the star wheel m of the shoe clearance adjustment device k. The shoe clearance adjustment device k, the adjustment lever l, and a spring n stretched between the adjustment lever l and the pivot lever i make up an automatic shoe clearance adjustment mechanism.
When the service brake is applied, the two brake shoes d, e spread open with the point of abutment with the anchor block c as the fulcrum, and this drum brake device functions as a leading-trailing type brake.
For activating the parking brake, the parking lever h is to be pulled and, the force of that action is transferred in sequence to the rod j, the pivot lever i, and the shoe clearance adjustment device k. The left shoe d spreads open, with its point of abutment with the anchor block c as the fulcrum, and frictionally engages the brake drum o. Then the pivot lever i spreads open, with its point of abutment with the shoe clearance adjustment device k as the fulcrum. The right brake shoe e, on which the pivot lever i is pivoted, spreads open simultaneously to frictionally engage the brake drum o. Moreover, the reaction force of the parking brake lever h is acting on the lower segment of the left brake shoe d. At this point, should the vehicle be stopped on an incline or decline, and torque is applied on the brake drum o in the direction of arrow R, the friction force of the left brake shoe d is transferred via the shoe clearance adjustment device k as a force to spread open the right brake shoe e. Should torque be applied in the opposite direction on the brake drum o, the friction force of the right brake shoe e is transferred via the shoe clearance adjustment device k to the left brake shoe d. As such, when the parking brake is applied, this drum brake device functions as a duo-servo type brake.
The conventional drum brake device uses a pivot sleeve p and an E ring q to pre-mount the pivot lever i onto the one brake shoe e as shown in FIG. 21. Several disadvantaqes result.
The extra components, namely, the pivot sleeve p and E ring q, required to pre-mount the pivot lever i onto the brake shoe e increase the cost of the device.
The pivot sleeve p is installed by utilizing a shoe hold mechanisms. As shown in FIG. 21, the pivot sleeve p is installed such that its flange r lies between the shoe web u and the bottom face of the plate spring t which forms part of the shoe hold mechanism s. Therefore, a different holding force would be generated on the left and right brake shoes d, e if the left and right shoe hold mechanisms were of the same dimensions, thereby increasing the resistance of the brake shoe e with attached pivot lever i to extend or contract, possibly causing the brake shoe to drag or creating other problems thereof.
To avoid the possible problems of potential dragging of brake shoe e, it is necessary to design the left and right shoe hold mechanisms to different specifications. For example, the length of the pin v comprising part of the right shoe hold mechanism s could be made longer than the pin v of the left shoe hold mechanism, or the plate springs t, t could be designed to different specifications, or the height of the drawn step of the backplate a on which the pin v is mounted could be increased. In any of these cases, a difference by an amount equal to the thickness of the flange r of the pivot sleeve p must be compensated. In other words, in order that the left and right shoe hold mechanisms perform uniformly, at the very least, the components of one mechanism would have to be built to the same shape but of different dimensions. The greater number of components coupled with different specifications for certain similar components would not only make the device more expensive but create chances of misassembling the device.
In handling the brake shoe e on which the pivot lever i is mounted, it can rotate freely until either the upper or lower segments of the pivot lever i abuts against the shoe rim. This is a nuisance when installing the shoe clearance adjustment device k and the rod j onto a drum brake device which is already mounted on the vehicle in the repair shop.